The present invention relates to a control device of a spark-ignition gasoline engine.
In improving a theoretical thermal efficiency of a spark-ignition gasoline engine, increasing a geometric compression ratio of the spark-ignition gasoline engine is effective. For example, JP2007-292050A discloses a high compression ratio spark-ignition direct injection engine in which a geometric compression ratio is set at 14:1 or above.
Further, as an art of achieving both an improvement in exhaust emission and an improvement in thermal efficiency, for example, as disclosed in JP2007-154859A, a combustion mode of compression-igniting lean air-fuel mixture is known. Increasing a geometric compression ratio in an engine where such a compression-ignition combustion is performed leads to increasing a pressure and a temperature at the end of compression stroke and, therefore, is advantageous in stabilizing a compression-ignition combustion.
Meanwhile, although a spark-ignition gasoline engine such as the one disclosed in JP2007-292050A is advantageous in improving the thermal efficiency, there has been a problem that when an operating state of the engine is especially within a low engine speed range and a middle or high engine load range, an abnormal combustion such as a pre-ignition and knocking (spark-knock) is easily caused.
Further, with an engine where a compression ignition combustion is performed, even if the compression-ignition combustion can be performed within an operating range where an engine load is low, as the engine load increases, the compression-ignition combustion turns into a combustion caused by a pre-ignition where an increase in pressure is rapid. Therefore, a combustion noise becomes louder and the abnormal combustion such as knocking is caused, at the same time, Raw NOx is increased due to a high temperature combustion temperature. Thus, as disclosed in JP2007-154859A, even with the engine where the compression-ignition combustion is performed, within the operating range where the engine load is high, the compression-ignition combustion is not performed but a spark-ignition combustion by operating an ignition plug is generally performed. However, with an engine where a geometric compression ratio is set high so that the compression-ignition combustion is stabilized, within an operating range where an engine speed is high, where a spark-ignition combustion is performed, a problem that an abnormal combustion is caused similarly to the engine in JP2007-292050A arises.
The present invention is made in view of the above situations and avoids an abnormal combustion in a spark-ignition gasoline engine with a high compression ratio in which a geometric compression ratio is set comparatively high, for example 14:1, when the engine is within a high load range.
An abnormal combustion such as a pre-ignition and knocking corresponds to a self-ignition reaction due to unburned air-fuel mixture being compressed during a compression stroke and a self-ignition reaction due to an unburned part of the air-fuel mixture being compressed by an expansion of a burned part thereof while the air-fuel mixture is burned. With the conventional engine where fuel is injected thereinto during an intake stroke, one of the factors that cause such an abnormal combustion is a long unburned air-fuel mixture reactable time, in other words, a time length from the start of the fuel injection until the end of combustion.
The reactable time of the unburned air-fuel mixture is configured by three periods: an injection period in which a fuel injection valve injects the fuel, an air-fuel mixture forming period starting from when the fuel injection completes until when a combustible air-fuel mixture is formed around the ignition plug, and a combustion period from when the combustion starts by the combustible air-fuel mixture being ignited around the ignition plug to when the combustion ends. Through studying a measure for shortening these respective three periods, it has been found that injecting the fuel into a cylinder at a timing near a compression top dead center with a comparatively high fuel pressure shortens the injection, air-fuel mixture forming, and combustion periods, respectively, and, as a result, leads to the completion of the present invention.